The invention relates to electrically heated aircraft floor panels made as composite material panels, particularly for use as a floor panel directly securable to a floor support grid structure of an aircraft. The panel comprises several layers and a lightweight core such as an open-cell or honeycomb core, and at least one step-on cover layer.
Conventional composite material panels including an electrical heater are known as sandwich panels and are referred to herein as composite panels which are multi-layer structures including a honeycomb, open-cell, or foam core bonded between upper and lower cover layers that are typically made of glass fiber and/or carbon fiber reinforced materials. The cover layers are bonded or laminated to the core in an autoclave under application of heat and pressure. The dimensions of the core and of the cover layers are selected according to the particular application or use and the strength required of the composite material panel. An example of a honeycomb sandwich structure without an electric heater is shown in the magazine xe2x80x9cFlight Internationalxe2x80x9d, Apr. 17, 1982, pages 988 and 989. Panels with an electric heater are disclosed in U.S. Pat. No. 2,512,875 (Reynolds) and in U.S. Pat. No. 3,697,728 (Stirzenbecher). These conventional panels leave room for improvement especially with regard to strength, durability and particularly safety requirements that must take into account the electrical heating of such composite panels to avoid fire hazards.
U.S. Pat. No. 3,961,157 (Miller et al.) discloses a lightweight electrically heated panel that can be a portable heater or it may be installed as a so-called modesty panel in a desk. The temperature is controlled by a thermostat connected in series with a thermal fuse. The thermostat and fuse are inserted in a honeycomb core where both are thermally insulated from the electrical heater. Such a structure cannot meet the strength and safety requirement of an aircraft floor panel.
Composite panels, particularly those with a honeycomb or open-cell core have a low weight and high strength and, for this reason, are particularly suitable as construction material for aircraft. Honeycomb sandwich panels are used, for example, as floor panels in aircraft. In a passenger cabin of an aircraft, such floor panels are covered either with a synthetic covering NTF, so-called non-textile floor covering, or with carpeting. In both instances, the panel causes fire safety problems due to the electrical heating.
The floor area near the aircraft door in a passenger cabin has to be heated during flight because the temperature on the outer skin of an aircraft flying at high altitudes can be as low as xe2x88x9255xc2x0 C. Therefore, the temperature in the floor area near the door can sink to as low as xe2x88x9215xc2x0 C., particularly after prolonged flight durations of approximately 5 hours or more at these altitudes. Passive measures to protect the door area against the cold, such as providing an insulated floor covering, are generally not adequate at such low temperatures and heat energy must be provided to these areas. For this reason, conventional warm air heating systems are used to feed warm air to the floor area near the door. The use of warm air systems in aircraft, however, is inefficient for a number of reasons. Such systems require a substantial electrical power supply and an alternating airflow from the air conditioning system. These systems also increase the weight of the aircraft and may cause discomfort among passengers and/or flight personnel because of uneven surface temperatures or drafty dry air circulation. Contamination of the air vents and the danger of blocking air vents by baggage are further disadvantages of warm air heating systems.
An alternative solution to the problem of providing heat in the floor area near the door in an aircraft passenger cabin is to screw separately heatable metal floor panels onto the conventional floor panels. This solution also has the disadvantages of adding weight to the aircraft, as well as increasing the high energy consumption. Heatable metal floor panels add approximately 60 kg per panel to the weight of the aircraft which must be avoided.
German Patent Publication DE-02 39 22 465 A1 discloses a panel component in which an electric heating device is embedded. The known panel is preferably constructed as a plywood panel and is typically used in housing construction and particularly in household areas such as the kitchen. The heating device is constructed of a plurality of tape-shaped electrodes and a radiant heater formed as a layer of synthetic material. Carbon particle mixtures are used as the radiant heater layer. Because of the combustion properties of such carbon particle mixtures, carbon particle heaters cannot be used in the passenger areas of an aircraft unless extensive safety measures are taken which add to the weight and cost of the aircraft. Thus, for reasons of safety and economy, such conventional systems are generally unsuitable for use in commercial passenger aircraft. These considerations also apply to the disclosures of the following Japanese Patent Publications: JP 4-136630, JP 2-61435, and JP 63-161328. Each of these publications relates to building construction and does not meet aircraft safety requirements.
In view of the above it is the aim of the invention to achieve the following objects singly or in combination:
to provide a composite electrically heated aircraft floor panel that can be used instead of a conventional unheated floor panel in the area next to and alongside an aircraft door in a passenger cabin or in other cold spaces in the floor area of an aircraft, in order to achieve homogenous surface temperatures at the floor level, preferably in the range +20xc2x0 C. to +35xc2x0 C. under normal operating conditions;
to construct such a panel so that it achieves a reduction in weight and in energy consumption, compared to conventional aircraft floor heating solutions;
to construct such a panel that it simultaneously satisfies the mechanical strength of a step-on floor panel and the electrical safety requirements that must be met by aircraft floor panels particularly against fire hazards;
to achieve a uniform floor heat distribution by radiation that emanates in a controlled manner per surface area unit over the surface area of the panel so that for example panel areas directly next to the door radiate more heat than areas of the same panel further away from the door;
to avoid generating uncomfortable air drafts in the door areas of a passenger aircraft; and
to provide the present panels with surface area configurations that conform to the floor layout.
An aircraft floor heating panel according to the invention comprises a heat resistant fiber composite core structure for providing the structural strength of an aircraft floor panel, an outer first section including an electrical heater preferably a foil heater and a heat distributing metal plate forming a walk-on surface, a first adhesive bond between the electrical heater such as a foil heater and the heat distributing metal plate, a second adhesive bond between the foil heater and the fiber composite core structure, an outer second section including one or more heat insulating pads bonded to the fiber composite core structure opposite the outer first section so that the fiber composite core structure is sandwiched between the outer first and second sections. One or several cut-outs are provided in the composite core structure below the heat distributing metal plate. A PTC temperature sensor is embedded in at least one cut-out in heat sensing contact or proximity with the heat distributing metal plate. The positive temperature coefficient sensor is electrically connected to a control and power supply unit in the aircraft for switching a power supply on and off under normal operating temperatures. Redundant fire hazard protection components are electrically connected in series with the electrical foil heater for interrupting an electrical power supply to the electrical foil heater thereby overriding any control by the temperature sensor in response to temperatures exceeding normal operating temperatures.
The redundant fire hazard preventing components preferably comprise a first thermostatic switch embedded in a preferably separate cut-out in the fiber composite core structure and responsive to a first cut-off temperature, a second thermostatic switch embedded in another separate cut-out and responsive to a second cut-off temperature higher than the first cut-off temperature, and an electrical circuit connecting the electrical foil heater, the first thermostatic switch and the second thermostatic switch in series with each other and to an electric power supply which is switched on and off in response to the above mentioned sensor. The second thermostatic switch opens the electrical power supply circuit when the first thermostatic switch failed to open the electrical power supply circuit and the second higher cut-off temperature is reached. Preferably, each thermostatic switch is embedded with moisture sealing, heat resistant potting material in its own recess in the fiber composite core structure, but in heat sensing proximity of the heat distributing metal plate. The second thermostatic switch may be constructed as a resettable thermostatic fuse.
According to the invention there is further provided an aircraft comprising a fuselage, at least one door in the fuselage, a floor support structure in the fuselage, at least one step-on floor heating panel secured to the floor support structure next to the door, an electrical heater such as a foil heater in the floor heating panel, a heat distributing metal plate forming a step-on surface as an integral part of the floor heating panel opposite the floor support structure, at least one temperature sensor in the floor heating panel positioned in a location away from the door for sensing a normal floor panel temperature, particularly the hottest panel temperature compared to the cabin temperature in the floor area near the door and with reference to the temperature conditions below the floor heating panel. The temperature sensor in the floor heating panel takes these different temperatures into account. Therefore, the temperature sensor is arranged away from the door sill which could still have a temperature that is lower than the normal operating temperature or passenger comfort temperature of about 25xc2x0 C. to 35xc2x0 C. in the door area. A power supply circuit is Controlled by the at least one temperature sensor in response to a temperature control signal generated by the at least One temperature sensor. The control signal causes a power supply switch to repeatedly open and close for maintaining a desired temperature which may be selected by adjusting the temperature sensor respectively. The temperature sensor cooperates or rather is overridden by a fire hazard protection device which preferably comprises a first and a second thermostatic switch connected in series with each other in the power supply circuit. The first and second thermostatic switches are positioned in the panel core structure in heat transfer contact or proximity with the heat distribution metal plate away from the door, wherein the first thermostatic switch responds to a first temperature in a first preferably adjustable range and the second thermostatic switch responds to a second temperature higher than the first temperature. Preferably, the second temperature or its range is also adjustable.
Preferably, for maintenance use a monitoring circuit is connected to the second thermostatic switch to make certain that the second thermostatic switch remains open until normal operating temperatures have been restored and a reset signal is supplied to the second thermostatic switch.
A particular advantage of the composite panel according to the invention is a substantial reduction in weight relative to conventional floor heater solutions. This is an essential feature of the invention, since weight reduction is a continuous concern in aircraft construction. The composite panel according to the invention uniformly heats the cold areas of the floor next to a door in an aircraft passenger cabin by providing an even distribution of heat without causing uncomfortably warm or drafty air flows.
The present film or foil heater is integrated into the composite panel by adhesively bonding flat foil heater elements directly between a heat distributing metal plate and a laminated lightweight heat resistant core structure for example by epoxy and/or acrylic adhesives, whereby the flat heater is sandwiched between the metal plate and the laminated core structure and the adhesive forms an electrical insulation between the electrical heater and the heat distributing metal plates. The laminated structure of the panel is constructed to provide the mechanical strength required for aircraft step-on floor panels. The heater and its controls are constructed to meet the stringent safety requirements applicable to electrical systems in an aircraft to avoid a fire in the aircraft.
The mechanical strength providing laminated structure comprises a lightweight honeycomb core sandwiched between two inner core protecting layers made of carbon fiber reinforced composite material. The core and CFC layers in turn are sandwiched between two mechanical strength providing glass fiber reinforced composite layers (GFC). This sequence of layers is necessary for obtaining the required protection of the lightweight core by the CFC-layers against deterioration and for obtaining the required mechanical core strength by the GFC-layers bonded to the core through the CFC-layers. The GFC-layers further function as electrical insulating layers to prevent any electrical contact between the CFC-layers and, for example, the heat distributing metal plate or any other metal contact with the floor structure. Further, the CFK layers are electrically grounded for avoiding so-called arc tracking.
The matrix material of the CFC- and GFC-layers is, for example a phenol-formaldehyde resin (PF). The heat distributing metal plate is preferably made of aluminum which provides a step-on surface and simultaneously a uniform even heat distribution over the surface area of the composite panel. The metal plate also protects the panel, particularly the heater against damage and mechanical stresses.
The above mentioned positive temperature coefficient (PTC) sensor in combination with the two thermostatic switches of the present composite panel provides a redundant safety protection against fire hazards. By providing temperature control signals to a control unit the PTC sensor maintains the cabin temperature next to the door in a normal range. At least one, preferably two separate safety thermostatic switches responsive to different temperatures and/or to an excess current, e.g. a short-circuit current, operate independently of the PTC sensor and override the PCT sensor if it fails to prevent an unusual temperature rise. The two safety switches are connected in series with each other in the power supply circuit of the floor panel electric foil heater. For safety reasons it is critical, that the safety switches work independently of each other and independently of the PTC sensor and that the safety switch responsive to the higher temperature or to an excess current can be independently monitored and controlled in its operation. These redundant temperature and/or current controls regulate the temperature of the floor panel to prevent overheating of the panel and thus to prevent a fire hazard.
At least one switch of the safety switches acts as an excess current control by opening the heater power supply circuit in response to an excess current which is also monitored in the power supply and control unit. A resettable fuse for example can switch off the heater power supply in response to a short-circuit. In case of damage to the panel the control unit switches off the power supply to the electric foil heater of the panel.
The position of the sensor and switches in the present composite panel is critical for obtaining a safe temperature control and for obtaining different heat outputs in different panel areas. For this purpose several cut-outs are preferably positioned in the composite panel for mounting the sensor and safety switches and respective conductors and terminals in the panel away from the aircraft door where the highest heat concentration per surface area unit is expected since the area closest to the door is the coldest. These cut-outs are covered with a watertight seal, preferably epoxy resin or similar potting material, to protect the panel, sensor, and switches against the ingress of moisture. It is critical that the PTC sensor and the thermostatic switches are positioned in heat sensing proximity of the heat distributing metal plate to assure that critical temperatures are positively controlled to maintain the required safety. A resettable fuse or over temperature switch responsive to an excess current may be positioned where it is most practical for the panel construction or production.
In a further embodiment of the composite panel according to the invention, an insulating layer, preferably of several synthetic foam pads, is positioned and bonded to the laminated core sandwich structure opposite the heat distributing metal plate. The insulating layer which is adhesively bonded to the bottom surface of the core sandwich structure has an areal or surface configuration which is smaller than the surface or a real configuration of the composite panel, whereby the panel is provided with mounting margins or rims and gaps for securing the panel to a floor support structure particularly a support grid structure. This insulating layer fits between the stringers and joists of the floor support structure, thereby reducing heat loss from the floor panel and thus reducing the amount of energy required for heating the floor next to and alongside an aircraft door.